1. Field of the Invention
The present invention relates to a method for in-situ extraction and chemical decontamination of contaminants in ground media comprising clayey materials.
2. Description of the Prior Art
Spills or accidents involving liquid contaminants being poured or dumped onto the ground can seep or leach the contaminants into the ground media. Depending on the specifics of the ground media, the contaminants generally settle in a layer of the ground media, co-mingled with soils, clayey materials or other ground media components. When possible, contaminants are removed through conventional drilling, pumping, displacing or other methods used to transfer liquid above the ground. Treating liquids that have already been extracted from the ground is generally considered to be much easier, and much less expensive, than attempting to remediate the contamination at the site, or in-situ. In many cases, however, contaminants cannot be separated from the ground media through these conventional processes. In such cases, in-situ treatment of the contaminants must be considered.
Conventional in-situ treatment technologies for cleaning contaminated subsurface media use injection ports or a combination of injection and extraction ports to deliver reagents and to extract reaction byproducts and contaminants. In-situ chemical oxidation or reduction requires the delivery of reagents in an aqueous medium. Following gravitation, the aqueous reagent solution administered to the subsurface through fixed injection ports becomes an integral part of the groundwater. The volume of contaminated subsurface media in the unsaturated zone above the groundwater table that is affected by the reagent solution is limited to the annular space of the injection ports. Within the groundwater, the reagent solution follows the natural or induced hydraulic gradient. The oxidizing and hydrophilic reagent solution follows preferred pathways, due to physical and chemical heterogeneities of subsurface media. Physical heterogeneities include variability in hydraulic conductivity caused by material changes, for example clay versus sand versus gravel soils versus fractured bedrock. Mineral surfaces are hydrophilic. The hydrophilic properties are altered by sorption of organic compounds such as natural soil organic matter and organic contaminants that contain both hydrophilic and hydrophobic moieties.
The physical constraints of conventional in-situ delivery systems and the physical chemical heterogeneities of ground media limit the effectiveness of oxidizing reagent solutions in making contact with contaminants. Moreover, the oxidizing reagents that are typically utilized in in-situ chemical oxidation systems, e.g. liquid hydrogen peroxide, sodium or potassium permanganate, sodium percarbonate, etc., can be unstable and/or short-lived.
Consumption of oxidant by matrix constituents typically exceeds the oxidant consumption by contaminants. To overcome these limitations, large volumes of highly concentrated reagent solutions are typically administered to the contaminated subsurface media. The introduction of highly concentrated and reactive solutions that contain non-specific oxidizing and/or reducing agents poses problems with respect to controlling the progress and the heat of these reactions.
In-situ oxidation systems are known that chemically oxidize organic contaminants to environmentally safe and non-toxic constituents. One such system is a reaction named after its discoverer, H. J. H. Fenton (1894). In this reaction, the oxidizing agent, hydrogen peroxide, is reacted with a metallic salt to generate free radicals with a higher oxidation potential than hydrogen peroxide. The free radicals react with organic compounds to either completely decompose them to carbon dioxide and water or to convert them to water soluble and biologically degradable compounds. A drawback to this process is that the catalytic decomposition of hydrogen peroxide and oxidation of organic compounds by radicals are both exothermic reactions.
A number of patents teach the art of treating contaminants with Fenton-type chemical systems in in-situ environments. The patents by Brown et al., U.S. Pat. No. 4,591,443, Vigneri, U.S. Pat. No. 5,520,483, Wilson, U.S. Pat. No. 5,611,642, Kelly et al., U.S. Pat. No. 5,610,065, and Cooper et al., U.S. Pat. No. 5,967,230, teach the introduction of liquid hydrogen peroxide and a metal catalyst, Fenton's Reagent, such as an iron salt, into the subsurface. Watts et al., U.S. Pat. No. 5,741,427, teaches the injection of a chelated metal catalyst for use in an in-situ chemical oxidation. All of the above-cited art adds a metal catalyst into the subsurface. In addition, the processes described in the above cited art include either the co-injection or the sequential introduction of reagents, where the oxidizing agent is added either before or after the metal catalyst. Finally, all of the above patents teach the necessity of introducing both the oxidizer and the metal catalyst together or separately into the subsurface to facilitate the oxidation of contaminants.
It should also be pointed out that the majority of sites are contaminated with multiple types of contaminants. Organic contaminants generally fall into several categories. These include contaminants composed of hydrogen and carbon atoms and are generally referred to as hydrocarbons. A second large cross section of contaminants is composed of hydrogen, carbon and halogen atoms and are known as halogenated compounds. This latter group of compounds is generally more recalcitrant than hydrocarbons.
Conventional methods of remediating halogenated compounds are the application of sodium or potassium permanganate, anaerobic reductive dechlorination and the application of nanoscale iron. While popular, these methods have serious complications that make them risky and generally require a long period of time if they are successful at all. Biological reductive dechlorination is dependent upon in-situ factors that will allow microbial proliferation. One of the most serious drawbacks to this technique is that it will not proceed where the concentrations of contaminants are in excess of the toxic threshold of the microbial community. Thus, it is not applicable to high concentrations of contaminants or conditions where free phase product is present. Similarly, although the application of nanoscale iron is not dependent upon biological factors, it is a solid suspension and, thus, extremely difficult if not impossible to inject in heavy soils such as hard clay. Therefore, the most popular method of application is trenching, which is expensive and requires the employment of heavy equipment and opening of the soil matrix, thereby exposing the contaminant to volatilization to the atmosphere. This practice can produce conditions unsafe for inhalation by site workers. Permanganate salts will successfully mitigate halogenated contaminants, but halogenated compounds are almost always co-contaminants of hydrocarbon compounds that cause permanganate to precipitate as manganese dioxide, thereby causing cessation of the oxidation reaction.
Furthermore, ground media comprising clayey materials presents additional challenges. Clayey materials (such as clay) may be relatively dense, in comparison to soil or silty materials. Clayey materials may also have both large and small passages, including a capillary-like pore structure that is generally not present in sand, soil or silty materials. While such pores allow the clayey material to hold liquids, including liquid contaminants, within them, such liquids are typically harder to remove from the ground media. The capillary pressure acts to retain liquid contaminants within them, while at the same time inhibiting contact between the contaminants and any fluid reagent which may be added. Furthermore, the size and multitude of the capillaries make it more difficult to reach the contaminants contained within them. Finally, a higher density ground media makes it more difficult to insert injection ports, or to drill, into the ground media for in-situ treatment—thus limiting the opportunities to contact the contaminants with a reagent.